1. Field of the Invention
This invention relates to a method for forming a deposited film, which comprises forming a photoconductive film, a semiconductor film or an insulating film on a substrate by utilizing light as the excitation energy, more particularly to a method for forming the deposited film containing, for example, silicon atoms on a certain substrate by creating a excited and decomposed state of a starting gas by imparting or utilizing light or light and, if desired, heat.
2. Description of the Prior Art
In the prior art, as the method for forming a deposited film of amorphous silicon (hereinafter written as "a-Si"), there have been known the glow discharge deposition method and the heat energy deposition method employing SiH.sub.4 or Si.sub.2 H.sub.6 as the starting material. That is, these methods comprise forming the deposited film of a-Si on the substrate by decomposing SiH.sub.4 or Si.sub.2 H.sub.6 as the starting material by electric energy or heat energy (excitation energy), and the deposited film has been utilized for various purposes as the photoconductive film, semiconductor, insulating film or the like.
However, in the glow discharge decomposition method wherein the deposited film is formed under high output discharging, it is difficult to control stably the conditions with reproducibility, because no uniform distribution state of uniform discharging can be obtained, and further the high output discharging has great effect on the film during film formation. As the result, it is difficult to ensure uniformity in electrical and optical characteristics as well as stability in quality of the film formed, and disturbance of the film surface during deposition and defects within the deposited film are liable to occur. Particularly, it has been very difficult to form a film with large area or great thickness having uniform electrical and optical characteristics according to this method.
On the other hand, also in the heat energy deposition method, since a high temperature of 400.degree. C. or higher is generally required, the substrate materials available are limited. Besides, since the probability of elimination of the useful bonded hydrogen atoms in a-Si is increased, desired film characteristics can be obtained with difficulty.
Accordingly, as one method of overcoming these problems, the photoenergy deposition method (hereinafter written as "photo-CVD") employing SiH.sub.4 or Si.sub.2 H.sub.6 as the starting material is recently attracting attention.
The photoenergy deposition method employs light as the excitation energy in place of glow discharge or heat in the above methods, and preparation of a deposited film of a-Si at a low energy level has been rendered practicable. Also, photoenergy can easily irradiate uniformly the starting gas and the film of high quality can be formed while maintaining uniformity with a lower energy consumption as compared with the deposition methods as described above. Also, the preparation conditions can easily be controlled to give stable reproducibility, and further the substrate is not required to be heated to a high temperature, whereby the scope of choice of the substrate can be broadened.
Whereas, in the photoenergy deposition method employing SiH.sub.4 or Si.sub.2 H.sub.6 as the starting material, decomposition with a dramatically good efficiency can be expected only to a limited extent. Accordingly, it has been pointed that film forming speed cannot be improved, thus posing a problem with respect to bulk productivity.